Candidate pathogenicity islands in the genome of ‘CandidatusRickettsiella isopodorum’, an intracellular bacterium infecting terrestrial isopod crustaceans

Abstract

The bacterial genusRickettsiellabelongs to the order Legionellales in the Gammaproteobacteria, and consists of several described species and pathotypes, most of which are considered to be intracellular pathogens infecting arthropods. Two members of this genus,R. grylliandR. isopodorum, are known to infect terrestrial isopod crustaceans. In this study, we assembled a draft genomic sequence forR. isopodorum, and performed a comparative genomic analysis withR. grylli. We found evidence for several candidate genomic island regions inR. isopodorum, none of which appear in the previously availableR. grylligenome sequence.Furthermore, one of these genomic island candidates inR. isopodorumcontained a gene that encodes a cytotoxin partially homologous to those found inPhotorhabdus luminescensandXenorhabdus nematophilus(Enterobacteriaceae), suggesting that horizontal gene transfer may have played a role in the evolution of pathogenicity inRickettsiella. These results lay the groundwork for future studies on the mechanisms underlying pathogenesis inR. isopodorum, and this system may provide a good model for studying the evolution of host-microbe interactions in nature.